Aboveground biomass and ecosystem carbon pools in tropical secondary forests growing in six life zones of Costa Rica

Abstract:

Carbon sequestration in tropical secondary forests growing in all climates must be quantified to understand their potential role in adaptation and mitigation strategies of global climate change. Total aboveground biomass (TAGB), soil carbon, and total ecosystem carbon (TEC) were measured in 54 secondary forests growing along a broad bioclimatic gradient of 6 life zones, from lowland Dry to Premontane Rain forests in Costa Rica. The potential of regenerating secondary forests to offset carbon losses due to climate change-mediated primary forest degradation into the year 2100 was also determined.
TAGB ranged from 12 Mg•ha⁻¹ (5 MgC•ha⁻¹) in a 9-yr Dry forest to 298 Mg•ha⁻¹
(143 MgC•ha⁻¹) in a 60-yr Wet forest. TAGB and carbon were correlated with forest age and the ratio of potential evapotranspiration to precipitation. Secondary forests were predicted to reach TAGB ≥ 90% of that in primary forest in 35 to 126 yrs. Mean soil carbon to 1 m ranged from 127 to 278 Mg•ha⁻¹ in the Dry and Premontane Rain life zones. There was no correlation of soil C pools with age.
TEC was as high as 440 Mg•ha⁻¹ in a 40-yr Premontane Rain forest. Maximum rates of TEC sequestration were highest (14 – 33 Mg•ha⁻¹•yr⁻¹) in life zones with intermediate levels of precipitation and lowest (7 and 12 Mg•ha⁻¹•yr⁻¹) in the Dry and Premontane Rain life zones. Secondary forests were predicted to attain ecosystem C levels similar to that of primary forests in 44 – 105 yrs.
Based upon simulations of forest growth and the changing land area covered by differing life zones due to climate change, carbon losses from primary forests ranged from < 6 to 65 Tg under two different climate change scenarios for the year 2100. Secondary forests would need to cover 19% of each life zone to offset the larger flux of carbon into the atmosphere. Our modeling did not include additional carbon losses associated with changing disturbance regimes and other climate change responses. Results from this study underscore the value of secondary forests for their potential to sequester carbon across a diversity of tropical climates as a means of climate change mitigation.